The following issues are addressed in this commit

(i)   pool_tags should be read into pool_tag map
(ii)  keys with small key_len intermittently "disappearing" when other keys are deleted
(iii) nvm_kv_get_pool_metadata needs to be compliant with documentation
(iv)  Global expiry thread does not run if the percentage of capacity used is same as last seen capacity
(v)   nvm_kv_get_pool_metadata return count > number of pools remaining